1. Field of the Invention
Exemplary embodiments of the invention relate to light emitting diodes.
2. Description of the Background
Gallium nitride (GaN) based light emitting diodes (LEDs) have been under development for about 10 years. GaN-based LEDs represent a significant change in LED technology and are used in a wide range of applications, including natural color LED display devices, LED traffic sign boards, white LEDs, etc. In the future, white LEDs are expected to replace fluorescent lamps, as the efficiency of white LEDs approaches the efficiency of typical fluorescent lamps.
A GaN-based light emitting diode is generally formed by growing epitaxial layers on a substrate, for example, a sapphire substrate, and includes an N-type semiconductor layer, a P-type semiconductor layer, and an active layer interposed there between. Further, an N electrode is formed on the N-type semiconductor layer, and a P electrode is formed on the P-type semiconductor layer. The light emitting diode is electrically connected to, and operated by, an external power source, through these electrodes. Here, electric current is directed from the P-electrode to the N-electrode, through the semiconductor layers.
Generally, since the P-type semiconductor layer has a high specific resistance, electric current is not evenly distributed in the P-type semiconductor layer. Instead, the current is concentrated on a portion of the P-type semiconductor layer having the P-electrode formed thereon, causing current concentration at an edge of the P-type semiconductor layer. The current concentration leads to a reduction in light emitting area, thereby reducing luminous efficacy. To solve such problems, a transparent electrode layer having a low specific resistance is formed on the P-type semiconductor layer, so as to enhance current distribution. In this structure, when supplied from the P-electrode, the electric current is dispersed by the transparent electrode layer before entering the P-type semiconductor layer, thereby increasing a light emitting area of the LED.
However, since the transparency of the transparent electrode layer is dependent upon the thickness thereof, the thickness of the transparent electrode layer is generally limited, thereby limiting the current dispersion. In particular, for a large area LED (having an area of about 1 mm2 or more and a high output), there is a limit to the current dispersion through the transparent electrode layer.
Meanwhile, the electric current flows into the N electrode through the semiconductor layers. Accordingly, the electric current concentrates on a portion of the N-type semiconductor layer adjacent to the N-electrode. That is, the current flowing in the semiconductor layers is concentrated in a region of the N-type semiconductor layer near the N-electrode is formed. Therefore, there is a need for a light emitting diode solving the problem of current concentration within the N-type semiconductor layer.
Typically, various types of electrode structures are used in a light emitting diode, to ensure uniform current dispersion. FIG. 1 illustrates a light emitting diode having a diagonal electrode structure. In FIG. 1, reference numeral 1 denotes an N electrode, 2 denotes a P electrode, 3 denotes an exposed N-type semiconductor layer, and 4 denotes a transparent electrode layer. Referring to FIG. 1, the diagonal electrode structure is highly effective for a small LED, but causes an increasing concentration of electric current on a central region of the LED, as the size of the LED increases, such that only the central region of the LED emits light. In addition, an electrode pattern of a simple facing-type structure also suffers from the same problems as the diagonal electrode structure.
FIG. 2 illustrates a light emitting diode having a combined electrode structure including a facing-type structure and a symmetrical extension-type structure, and FIG. 3 is a cross-sectional view taken along line A-A′ of FIG. 2. In FIG. 2 and FIG. 3, reference numeral 11 denotes a substrate, 13 denotes an N-type semiconductor layer, 15 denotes an active layer, 17 denotes a P-type semiconductor layer, 19 denotes a transparent electrode layer, 21 denotes an N electrode, 22 and 23 denote extensions of the N electrode, 31 denotes a P electrode, and 32 and 33 denote extensions of the P electrode.
Referring to FIG. 2 and FIG. 3, the combined electrode structure is generally used for large LEDs. It can be appreciated that the extension 22, 23, 32, 33 are formed over a light emitting area of an LED chip, and have an increased area for ensuring uniform current distribution over the light emitting area.
However, since the N-type semiconductor layer 13 is exposed by mesa-etching used to form the extensions 32, 33 of the P electrode 31 and the extension parts 22, 23 of the N electrode 21, the light emitting area is inevitably decreased. Moreover, in the current state of the art, the number of electrode pads formed on a single chip is more than doubled, for current diffusion, and a mesa-etching area for forming electrodes and extension parts of these electrodes is also expanded. The expansion of the mesa-etching area, caused by the increase in the number of electrode pads, results in a decrease in the light emitting area, based on the same chip area, thereby reducing light emitting efficiency.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.